Millable bridge plug system

ABSTRACT

A millable bridge plug system includes a mandrel, an upper cap end, a sealing member, ring members, cone assemblies, slip devices and a lower cap end. The sealing member, ring members, cone assemblies and slip devices are positioned on and around the mandrel. Ring members abut against an upper end and a lower end of the sealing member. The other sides of the ring members abut against the cone assemblies, and the cone assemblies engage respective slip devices. The upper cap end attaches to the first slip device, and the lower cap end attaches to the second slip device. The lower cap end holds the mandrel and second slip device together to resist rotation during milling.

CROSS-REFERENCE TO RELATED APPLICATIONS

See Application Data Sheet.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

THE NAMES OF PARTIES TO A JOINT RESEARCH AGREEMENT

Not applicable.

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC OR AS A TEXT FILE VIA THE OFFICE ELECTRONIC FILING SYSTEM (EFS-WEB)

Not applicable.

STATEMENT REGARDING PRIOR DISCLOSURES BY THE INVENTOR OR A JOINT INVENTOR

Not applicable.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a downhole tool for isolating zones in a wellbore. More particularly, the present invention relates to a millable bridge plug system.

2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.

A bridge plug is a downhole tool that is lowered into a wellbore. At a particular distance through the wellbore, the bridge plug is activated. The bridge plug opens and locks to seal the bridge plug to the walls of the wellbore. The bridge plug separates the wellbore into two sides. The upper portion can be cemented and tested, separate from the sealed lower portion of the wellbore. Sometimes the bridge plugs are permanent, and they seal an entire portion of the wellbore. Other times, the bridge plugs must be removed, and still other times, the bridge plugs must be removed and retrieved. These removable bridge plugs are millable or drillable, so that a drill string can grind through the bridge plug, making remnants of the destroyed bridge plug to remain at the bottom of a wellbore or to be retrieved to the surface by drilling mud flow.

Bridge plugs generally include a mandrel, a sealing member placed around the mandrel, ring members adjacent the end of the sealing member and around the mandrel, upper and lower slip devices at opposite ends of the mandrel, and respective upper and lower cone assemblies engaged to the upper and lower slip devices. FIG. 1 shows the prior art bridge plug system 10 with a mandrel 12, sealing member 14, and upper and lower slip devices 16 and 18 shown. The bridge plug is placed in the wellbore by a setting tool on a positioning assembly, such as wireline, coiled tubing or even the drill string itself. Once in position at the correct depth and orientation, the bridge plug is activated. The setting tool holds the mandrel 12 in place, while a ramming portion of the setting tool exerts pressure on the stack, which includes the sealing member 14 and the slip devices 16 and 18. The end 22 has a cap which prevents the stack from sliding off the mandrel 12, when the ramming portion of the setting tool hits the stack. Instead, the pressure of the ramming portion compresses the stack, forcing the sealing member 14 to radially extend outward to seal against the wellbore or case and to flatten to a smaller height along the mandrel. The slip devices 16 are toothed and are distended radially outward by the stack to dig into the wellbore walls, locking the sealed configuration of the stack. The lower slip device 18 holds position by the cap at the end 22, while the upper slip device 16 lowers and locks the seal of the spread sealing member 14. When the ramming portion has compressed and locked the stack, the end 20 proximal to the setting tool on the positioning assembly is sheared, separating the bridge plug from the setting tool and the positioning assembly. FIG. 2 shows the prior art bridge plug system 10 in an activated and set state. Pressure on the lower cone assembly against the lower slip device 18 at the distal end of the mandrel causes the lower slip device 16 to open and latch against the wellbore. Continuing pressure by the ram expands the sealing member 14 against the rings to form a seal against the walls of the wellbore. Pressure on the upper cone assembly causes the upper slip device 18 to also open and latch against the wellbore, setting the seal of the sealing member.

A problem of the conventional bridge plug is the stabilization of the bridge plug during removal. A removal assembly, such as a drill string or other wireline device, has a drill element to drill through a millable bridge plug, the bridge plug must be able to resist rotation of the drill element itself. Otherwise, a partially milled bridge plug could become lodged on the tip of the drill element of the removal assembly. These remnants of the bridge plug would be rotating along with the drill element of the removal assembly, so that these last remnants could avoid being destroyed and possibly hinder further action of the drill element on bridge plugs further down the wellbore. The structures of the bridge plug are not milled for removal under the same conditions. The upper slip device is milled, then the upper cone assembly, and then the sealing member, etc., as the drill element travels downward through the wellbore. Once the upper slip device is milled, the remaining elements are the remnants holding the bridge plug in place. Once the upper slip device and the upper cone assembly are milled, there are fewer remnants holding the bridge plug in place. As elements are removed, fewer and fewer elements resist the rotation of the drilling element. There is a need to improve the bridge plug to resist the rotation of the drill element, even as the number of remnants decreases.

Conventional materials of the millable bridge plug, like all downhole tools, must withstand the range of wellbore conditions, including high temperatures and/or high pressures. High temperatures are generally defined as downhole temperatures generally in the range of 200-450 degrees F.; and high pressures are generally defined as downhole pressures in the range of 7,500-15,000 psi. Other conditions include pH environments, generally ranging from less than 6.0 or more than 8.0. Conventional sealing elements have evolved to withstand these wellbore conditions so as to maintain effective seals and resist degradation.

Metallic components have the durability to withstand the wellbore conditions, including high temperatures and high pressures. However, these metallic components are difficult to remove. De-activating and retrieving the bridge plug to the surface is costly and complicated. Milling metallic components takes time, and there is a substantial risk of requiring multiple drilling elements due to the metallic components wearing or damaging a drilling element of a removal assembly.

Non-metallic components are substituted for metallic components as often as possible to avoid having so much metal to be milled for removal of the bridge plug. However, these non-metallic components still must effectively seal an annulus at high temperatures and high pressures. Composite materials are known to be used to make non-metallic components of the bridge plug. These composite materials combine constituent materials to form a composite material with physical properties of each composite material. For example, a polymer or epoxy can be reinforced by a continuous fiber such as glass, carbon, or aramid. The polymer is easily millable and withstands the wellbore conditions, while the fibers also withstand the wellbore conditions and resist degradation. Resin-coated glass is another known composite material with downhole tool applications. Composite materials have different constituent materials and different ways of combining constituent materials.

It is an object of the present invention to provide an embodiment of the millable bridge plug system.

It is another object of the present invention to provide an embodiment of the millable bridge plug system with improved slip devices and cap ends.

It is another object of the present invention to provide an embodiment of the millable bridge plug system with improved slip devices to resist rotation of a drilling element of a removal assembly.

It is still another object of the present invention to provide an embodiment of the millable bridge plug system with slip devices having attachments to respective end caps to resist rotation.

It is yet another object of the present invention to provide an embodiment of the millable bridge plug system with slip devices engaged to respective end caps to resist rotation, when the bridge plug system is milled for removal.

These and other objectives and advantages of the present invention will become apparent from a reading of the attached specifications and appended claims.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention include a millable bridge plug system with a mandrel, an upper cap end, a sealing means positioned around the mandrel, a plurality of ring members, a plurality of cone assemblies, a plurality of slip devices, and a lower cap end. The elements are mounted on the mandrel. The sealing means has an upper end and a lower end in the middle of the system. A first ring member is placed adjacent the upper end of the sealing means, and a second ring member is adjacent the lower end of the sealing means. A first cone assembly is proximate to the first ring member, and a second cone assembly is proximate to the second ring member. The slip means extend radially outward and engage an inner surface of a surrounding borehole to lock the position of the bridge plug. The upper cap end attaches to the mandrel and the first slip means, and the lower cap end attaches to the mandrel and the second slip means. The cap ends resist rotation of a removal assembly during a milling operation.

Both the upper cap end and the lower cap end connect to the mandrel and respective slip means. The embodiments of the present invention relate to the lower cap end attached to the second slip means because the upper cap end and the first slip means are milled before other parts of the bridge plug system. Thus, these milled structures do not resist rotation of the system, even though they could in the beginning of the milling. As a practical matter, the lower cap end is a focus of the present application.

Embodiments of the attachment of the lower cap end and the second slip means include the lower cap end being comprised of a male connector means, and the second slip means being comprised of a female connector means. The male-female connection is oriented so that the connectors face each other, when assembled. The male-female connection is a removable engagement, such that the lower cap end and the second slip means can be attached and removed, during assembly. The type of removable engagement includes slide-fit engagement, groove-fit engagement, friction-fit engagement, and snap-fit engagement. The male connector means should have a shape complementary to a shape of the female connector means. The male-female connection also includes structures to prevent lateral movement between the lower cap end and the second slip means.

Alternate embodiments include the male connector means as removable from the lower cap end. The protruding male connector can be mounted in the lower cap end as a separate piece or peg. In versions with more than one connector, the connectors are distributed along the perimeters of the lower cap end and the second slip means. These connectors can also be evenly distributed along the perimeter.

The method of milling the bridge plug system, according to embodiments of the present invention, include forming a bridge plug with the lower cap end attached to the mandrel and the second slip device, installing the bridge plug in a wellbore, and drilling with a removal assembly. The removal assembly mills the upper cap end, upper portion of the mandrel, the first slip device, the ring members, the sealing member, and the cone assemblies, while the second slip device, lower cap end, and lower portion of the mandrel hold position. The anchoring of the second slip device before milling stabilizes the system, while the system is being dismantled.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic view of a prior art bridge plug system, being placed in a wellbore.

FIG. 2 is another schematic view of the prior art bridge plug system, being locked in position within the wellbore.

FIG. 3 is a perspective view of an embodiment of the bridge plug of the present invention.

FIG. 4 is an exploded perspective view of the embodiment of FIG. 3.

FIG. 5 is a cross-sectional view of an embodiment of the bridge plug of the present invention along an axis of the bridge plug, showing placement in the wellbore.

FIG. 6 is a cross-sectional view of an embodiment of the bridge plug of the present invention along an axis of the bridge plug, showing an activated configuration in the wellbore.

FIG. 7 is a perspective view of a slip device of an embodiment of the bridge plug of the present invention for one connector.

FIG. 8 is an alternate perspective view of an embodiment of the slip device according to FIG. 7.

FIG. 9 is a perspective view of an embodiment of the lower cap end of FIGS. 3 and 4, showing more than one connector.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 3-6, an embodiment of the millable bridge plug system 100 of the present invention is shown. The system 100 includes a mandrel 112, an upper cap end 128, a sealing means 114, and a plurality of ring members, 116, 118, a plurality of cone assemblies 120, 122, a plurality of slip means 124, 126, and a lower cap end 130. The sealing means 114, ring members 116, 118, cone assemblies 120, 122 and the slip means 124, 126 are stack structures mounted on the mandrel 112, sharing a common radial axis of alignment. FIGS. 3-6 also show an upper cap end 128 and a lower cap end 130. The millable bridge plug system 100 is placed within a wellbore or borehole of a well by a setting tool. The wellbore or the borehole could have a casing or not, and the orientation of the wellbore is variable. FIG. 5 shows an embodiment with a casing 132. The bridge plug system 100 can be used in all ranges from generally vertical to generally horizontal orientations. As previously described, the millable bridge plug system 100 is used to isolate zones within the wellbore, separating sections of the wellbore for production or isolation. The system 100 is millable or drillable, such that a removal assembly, such as a drill string, can be used to grind through the system 100. All of the components of the system 100 are destroyed so that the isolated zone of the wellbore is removed.

The mandrel 112 of the system 100 is a generally tubular member formed of a material to withstand the heat and pressure of the borehole conditions. The mandrel 112 is also millable. The mandrel 112 may have a bridge 134, which seals the zone above the system 100 from the zone below the system 100. The sealing means 114 is positioned around the mandrel 112. The sealing means 114 has an upper end 136 and lower end 138 as shown in FIGS. 5 and 6. The sealing means 114 is generally symmetrical to start and is comprised of a deformable material.

FIGS. 3-6 also show the plurality of ring members, 116, 118. There is a first ring member 116 adjacent the upper end 136 of the sealing means 114 and a second ring member 118 adjacent the lower end 138 of the sealing means 114. The ring members 116, 118 surround the sealing means 114 and surround the mandrel 112. The ring members 116, 118 contact the sealing means 114 and can exert pressure on the sealing means 114. In an activated state, the system 100 has the sealing means 114 compressed to radially extend to contact the wellbore or casing 132. The ring members 116, 118 directly contact the sealing means 114. The seal created by the sealing means 114 isolates the zones on the wellbore. In combination with the bridge 130 in the mandrel 112, the wellbore is separated.

The system 100 also includes the plurality of cone assemblies, 120, 122. FIGS. 3-6 show a first cone assembly 120 proximate to the first ring member 116 and a second cone assembly 122 proximate to the second ring member 118. As shown in exploded view of FIG. 3, the first ring member 116 is mounted on the mandrel 112 between the first cone assembly 120 and the sealing means 114. Similarly, the second ring member 118 is mounted on the mandrel 112 between the second cone assembly 122 and the sealing means 114. The cone assemblies 120, 122 contact the ring members 116, 118 and can exert pressure on the ring members 116, 18. In an activated state, the system 100 has pressure of the cone assemblies 120, 122 pushing through the ring members 116, 118 to the sealing means 114.

FIGS. 3-6 also show the plurality of slip means 124, 126 for extending radially outward and engaging an inner surface of a surrounding borehole. The slip means 124, 126 lock the position of the system 100 by fixedly engaging the casing 132 or other structure on the inner surface of the borehole. The slips dig into the casing 132 to anchor the millable bridge plug system 100. Pressure can be exerted on the system 100 to create the seal with the sealing means 114, once the slip means 124, 126 are active or while the slip means 124, 126 are being activated. There is a first slip means 124 mounted around the mandrel 112 and engaging the first cone assembly 120 and a second slip means 126 mounted around the mandrel 112 and engaging the second cone assembly 122. The present invention may include further stack structures, such as cone seats or other supplemental ring members. Embodiments of the present invention relate to the structures and interactions between particularly defined stack structures to properly control the force exerted by the setting tool during installation.

FIGS. 7-9 show detailed views of embodiments of the attachment of the lower cap end 130 and the second slip means 126. FIGS. 7 and 8 show alternate perspective views of the second slip means 126 for one male-female connection. The male-female connection is shown in FIGS. 3, 4 and 7-8. Similar to the first slip means 124, the second slip means 126 is stacked on the mandrel 112. The mandrel 112 aligns all of the elements onto a single axis. The mandrel 112 inserts through the second slip means 126. As shown in FIGS. 5-6, the mandrel 112 attaches to the lower cap end 130 with a portion 194 engaging the body of the mandrel 112 in the stacking formation of the other element. In some embodiments, the fixed engagement can be screw-fit engagement with complementary threaded ends locking together between the lower portion of the mandrel 112 and the lower cap end 130. The fixed engagement can be friction fit or even welding or adhesives to make the lower cap end 130 integral with the lower portion of the mandrel 112. The fixed engagement between the mandrel 112 and the lower cap end 130 does not permit rotation of the mandrel 112 relative to the lower cap end 130. FIG. 9 does not show the mandrel 112 to lower cap end 130 fixed engagement because the connection is internal to the lower cap end 130, consistent with FIGS. 5 and 6. Also, FIG. 8 shows more than one male-female connection.

FIGS. 3-6 also show the lower cap end 130 attaching to the second slip means 126 on a side opposite from the second cone assembly 122. The second slip means 126 is sandwiched between the second cone assembly 122 and the lower cap end 130.

Embodiments of the second slip means 126 and lower cap end 130 are shown in FIGS. 7-9. The second slip means 126 is comprised of a plurality of extendable blades 152 arranged around a cylindrical body 154. The tips 156 of the blades face the second cone assembly 124, as consistent with FIGS. 5 and 6. The side 160 facing the lower cap end 130 is comprised of a female connector means 158. The lower cap end 130 has a contact surface 162 facing the second slip means 126. The contact surface 162 can be separate from the fixed engagement of the mandrel 112, also consistent with FIGS. 5 and 6. The lower cap end 130 is comprised of a male connector means 164 protruding from the contact surface 162. The male-female connection is oriented so that the connectors 158, 164 face each other, when assembled.

The male-female connection can be a removable engagement, such that the lower cap end 130 and the second slip means 126 are separable and attachable, during assembly. Embodiments of the present invention include the removable engagement as slide-fit engagement, groove-fit engagement, friction-fit engagement, or snap-fit engagement. FIGS. 3-4 show a slide-fit engagement. FIGS. 7-9 also show a slide-fit engagement. The female connector means 158 is a slot, and the male connector means 164 is a protrusion. The protrusion slides into the slot during assembly. There is no rotational movement or lateral movement relative to each other between the second slip means 126 and the lower cap end 130.

The present invention also includes a male connector means 164 with any shape complementary to a shape of the female connector means 158. FIGS. 3-4 and 7-9 show a rectangle; however, any suitable shape, such as rectangular or trapezoidal, could be complementary for whichever type of removable engagement. The shape selection can be specific between a particular set of second slip devices 126 and lower cap ends 130 or modular so that any second slip device 126 can lock to any lower cap end 130. The lower cap end 130 and the second slip means 126 are both cylindrical, such that the shape of the male and female connector means 158, 164 can be radially oriented or arranged around the circular perimeter of the respective side 160 and contact surface 162. In some embodiments, the wider portion of the shape sets the radial orientation. The wider portion faces toward the circular perimeter with the larger diameter, like truncated pie wedges for trapezoidal shapes.

Other embodiments, including the embodiment shown in FIG. 9, show the male connector means 164 comprised of a locking shoulder 166. The locking shoulder 166 is formed by a T-shape protrusion 172 of the male connector means 164 on the lower cap end 130. The complementary T-slot 168 of the female connector means 158 is shown in FIG. 7. The complementary T-slot 168 has a slot shoulder 170 engaging the locking shoulder 166. In this embodiment, there is no lateral movement between the second slip means 126 and the lower cap end 130. The rotational movement relative to each other is also prevented.

One embodiment shown in FIGS. 3, 4 and 9 is a male connector means 164 comprised of a post 174 and a head 176 at an end of the post 174. A basic T-shape is not the only embodiment of the present invention, but the T-shape is an illustrative example. The post 174 protrudes from the contact surface 162 of the lower cap end 130 toward the second slip means 126. The relevant complementary shape is now the shape of the head 176 or shape of the post 174 and head 176, depending upon the shape of the female connector means 158.

An alternate embodiment is a removable male connector means 164. The male connector means 164 can be attached and removed from the contact surface 162 of the lower cap end 130. The protruding male connector means 164 can be a separate piece or peg. In this embodiment, the shape remains complementary to the female connector means 158. A head-post, T-shape, trapezoidal, or locking shoulders are embodiments of the removable male connector means 164. The removable male connector means 164 can be an advantage in assembly of the system 100, while preserving the relative locked rotational and lateral movement. Even as a separate element, the male connector means 164 is made integral with the lower cap end 130 through mechanical engagement, which is removable. The mechanical engagement of the male connector means 164 to lower cap end includes slide-fit engagement, groove-fit engagement, friction-fit engagement, and snap-fit engagement. The male connector means 164 may be fit into the female connector means 158 of the second slip means 126 and a slot or groove or cavity on the lower cap end 130.

In embodiments with more than one connector, the male and female connector means 158, 164 are distributed along the perimeters of the lower cap end 130 and the second slip means 126. FIG. 9 shows more than one connector with both connectors evenly distributed around the lower cap end 130. With the lower cap end 130 and second slip means 126 being generally cylindrical, the male and female connector means 158, 164 are arranged in rings or a circular perimeter of the lower cap end 130 and second slip means 126. For example, when the male connector means is comprised of two protrusions, then the two protrusions are placed across from each other on the ring of lower cap end 130. With three protrusions, the three protrusions are placed 120 degrees from each other on the ring or perimeter. The distribution can be even along the ring. In other embodiments, a particular side can be strengthened with more than one protrusion on a particular arc of the ring.

FIGS. 3 and 4 show an alternate embodiment with the upper cap end 128 attached to the mandrel 112 and the first slip means 124. The first slip means 124 is comprised of a plurality of extendable blades 178 arranged around a cylindrical body 180. The tips 182 of the blades 178 face the first cone assembly 120, as consistent with FIGS. 5 and 6. The side 184 facing the upper cap end 128 is comprised of a female connector means 186. The upper cap end 128 has a contact surface 188 facing the first slip means 124.

In the alternate embodiment of FIGS. 3 and 4, the upper cap end 128 attaches to the first slip means 124 analogous to the lower cap end 130 attaching to the second slip means 126. There can be a female connector means 186 in the first slip means 124 and a male connector means 190 on the upper cap end 128. The removable attachment includes slide-fit engagement, groove-fit engagement, friction-fit engagement, and snap-fit engagement, for complementary shapes between the female connector means 190 of the first slip means 124, and the male connector means 190 of the upper cap end 128. The alternate embodiment provides additional stability against rotation of elements on the mandrel 112 and prevents lateral movement between the elements in the stack. The lock of the upper cap end 128 does not improve much of the milling operation because the upper cap end 128 is one of the first elements drilled by the removal assembly. However, the male-female connection has benefits during assembly and installation of the system.

Embodiments of the present invention include the method of milling a bridge plug system 100. The bridge plug system is formed by assembling the upper cap end 128 onto the upper portion of the mandrel 112 and inserting the mandrel 112 through the first slip device 124, the first cone assembly 120, the first ring member 116, the seaing member 114, the second ring member 118, the second cone assembly 122, the second slip means 126, and the lower cap end 130. The elements are stacked along the mandrel 112. The lower cap end 130 is attached to both the second slip means 126 and the mandrel 112. The attachment between the lower cap end 130 and the second slip means 126 is more than the stacking arrangement. Then, the bridge plug system 100 is installed by placing the bridge plug system 100 in a wellbore, forming the seal on the wellbore, and locking the system 100 in position within the wellbore.

In some embodiments, the system 100 is lowered into the wellbore having inner walls, such as a casing, using a setting tool on a positioning assembly. The mandrel is held in place as the stack structures 114, 116, 118, 120, 122, 124, and 126 are hammered by a ram portion of the setting tool. Pressure on the bridge plug system 110 forms a seal, when the sealing means 114 is compressed to radially extend outward to seal against the inner walls of the borehole. The ring members 116, 118 push the sealing means 114 to expand, and the cone assemblies 120, 122 push the ring members 116, 118. The cone assemblies 120, 122 also push the slip means 124, 126 to extend radially outward to fixedly engage the inner walls, locking the system 100 in position within the wellbore. At least one slip means 124, 126 is activated, so that stack structures are locked in the sealed position. The exerted pressure through the system 100 is controlled by the first means 140 and second means 142 on the sealing means 114, and sometimes in conjunction with the first ring means 144 and the second ring means 146 on the ring members 116, 118.

The method of milling the bridge plug system further includes drilling with a removal assembly through the bridge plug 100. When the bridge plug 100 has served its purpose and requires removal, a drill on the removal assembly mills through the stacked elements in order from top to bottom, including the upper cap, the upper portion of the mandrel, the sealing member, the ring members, the cone assemblies, and the first slip device. In the present invention, the second slip device, the lower cap end, and the lower portion of the mandrel are held in position without rotation or lateral movement relative to each other. The removal assembly can more easily drill through the remnants of the bridge plug, when the remnants are prevented from rotating by the second slip means 126. The improved milling happens until the second slip means 126 is milled itself.

Embodiments of the method include the step of forming the bridge plug system 100 by attaching the lower cap end 130 to the second slip means 126 with a male-female connection. The lower cap end 130 is comprised of a male connector, and the second slip device 126 is comprised of a female connector. The step of forming is further comprised of attaching the female connector and the male connector in at least one engagement of a group consisting of: slide-fit engagement, groove-fit engagement, friction-fit engagement, and snap-fit engagement. In embodiments of the invention, the shapes of the male and female connectors are complementary, and some embodiments have the male connector as removable from the lower cap end 130. The step of forming may also include forming the male connector on the lower cap end 130 by slide-fit engagement, groove-fit engagement, friction-fit engagement, or snap-fit engagement of the male connector to the lower cap end 130.

The present invention provides an embodiment of a millable bridge plug system. The slip devices and cap ends are innovative with unique attachment elements to provide functionality beyond the prior art. The problem of milling remnants of a bridge plug has not been addressed by prior art systems. The embodiments of the present invention are an inventive solution for stabilizing remnants during this removal of the bridge plug by milling. The slip devices are in an anchored position when installed. The blades have engaged the borehole walls for fixed placement of the bridge plug. The stability of the installation of the bridge plug can be utilized, during milling of the bridge plug, when the structures of the present invention are incorporated into slip devices and cap ends. The resistance to rotational and lateral movement of an installed bridge plug becomes resistance to rotational and lateral movement of a partially destroyed bridge plug, during a milling operation to remove the bridge plug. The structures of the slip devices and cap ends of the present invention enable the resistance of the blades engaged in the borehole wall to be used throughout all of the stacked elements of the bridge plug, even as each element is being milled. The present invention prevents loose remnants from spinning on the drilling element of the removal assembly without being milled. These loose remnants can cause damage and hinder the removal assembly, when milling bridge plugs further down the wellbore. The present invention provides a cleaner and more complete milling operation of installed bridge plugs.

The foregoing disclosure and description of the invention is illustrative and explanatory thereof. Various changes in the details of the illustrated structures, construction and method can be made without departing from the true spirit of the invention. 

We claim:
 1. A millable bridge plug system, comprising: a mandrel having an upper portion and a lower portion; an upper cap end attached at said upper portion of said mandrel; a sealing means positioned around the mandrel between said upper portion and said lower portion; a plurality of ring members, a first ring member adjacent an upper end of said sealing means and a second ring member adjacent a lower end of said sealing means; a plurality of cone assemblies, a first cone assembly proximate to said first ring member and a second cone assembly proximate to said second ring member, said first ring member being between said first cone assembly and said sealing means, said second ring member being between said second cone assembly and said sealing means; a plurality of slip means for extending radially outward and engaging an inner surface of a surrounding borehole, a first slip means mounted around said mandrel and engaging said first cone assembly and a second slip means mounted around said mandrel and engaging said second cone assembly; and a lower cap end attached at said lower portion of said mandrel, wherein said upper cap end attaches to said first slip means on a side opposite from said first cone assembly, and wherein said lower cap end attaches to said second slip means on a side opposite from said second cone assembly.
 2. The bridge plug system according to claim 1, wherein said lower cap end is comprised of a male connector means, and wherein said second slip means is comprised of a female connector means, said female connector and said male connector means attaching said lower cap end to said second slip means.
 3. The bridge plug system according to claim 2, wherein said female connector means and said male connector means attach in at least one engagement of a group consisting of: slide-fit engagement, groove-fit engagement, friction-fit engagement, and snap-fit engagement.
 4. The bridge plug system according to claim 2, wherein said male connector means has a shape complementary to a shape of said female connector means.
 5. The bridge plug system according to claim 4, wherein said shape of said male connector means is at least one of a group consisting of: rectangular and trapezoidal.
 6. The bridge plug system according to claim 4, wherein said shape of said male connector means has a wider portion oriented radially towards an outer surface of said lower cap end.
 7. The bridge plug system according to claim 4, wherein said shape of said male connector means forms a locking shoulder abutted against said shape of said female connector means, so as to prevent lateral movement between said second slip means and said lower cap end.
 8. The bridge plug system according to claim 2, wherein said male connector means is comprised of a post and a head at an end of said post, said post protruding from an end surface of said second slip means toward said lower cap end, said head having a shape corresponding to a shape of said female connector means.
 9. The bridge plug system according to claim 8, wherein said post is comprised of a slot on said end surface of said lower cap end and a connector member, wherein said head is positioned at an end of said connector member, said connector member being removably engaged to said slot, said connector member protruding from said slot in said end surface of said lower cap end toward said second slip means, said head having a shape corresponding to a shape of said female connector means.
 10. The bridge plug system according to claim 9, wherein said connector member and said slot being in removable engagement according to at least one of a group consisting of: slide-fit engagement, groove-fit engagement, friction-fit engagement, and snap-fit engagement.
 11. The bridge plug system according to claim 8, wherein said post and head form a locking shoulder, said locking shoulder abutting against said shape of said female connector means, so as to prevent lateral movement between said second slip means and said lower cap end.
 12. The bridge plug system according to claim 1, wherein said lower cap end is comprised of a male connector means arranged around a perimeter of said lower cap end, and wherein said second slip means is comprised of a female connector means arranged around a perimeter of said second slip means, said female connector and said male connector means being aligned around respective perimeters to attach said lower cap end to said second slip means.
 13. The bridge plug system according to claim 12, wherein said female connector means and said male connector means are distributed evenly and correspondingly around respective perimeters.
 14. A millable bridge plug system comprising: a mandrel having an upper portion and a lower portion; an upper cap end attached at said upper portion of said mandrel; a sealing means positioned around the mandrel between said upper portion and said lower portion; a plurality of ring members, a first ring member adjacent an upper end of said sealing means and a second ring member adjacent a lower end of said sealing means; a plurality of cone assemblies, a first cone assembly proximate to said first ring member and a second cone assembly proximate to said second ring member, said first ring member being between said first cone assembly and said sealing means, said second ring member being between said second cone assembly and said sealing means; a plurality of slip means for extending radially outward and engaging an inner surface of a surrounding borehole, a first slip means mounted around said mandrel and engaging said first cone assembly and a second slip means mounted around said mandrel and engaging said second cone assembly; and a lower cap end attached at said lower portion of said mandrel, wherein said upper cap end attaches to said first slip means on a side opposite from said first cone assembly, wherein said lower cap end attaches to said second slip means on a side opposite from said second cone assembly, wherein said lower cap end is comprised of a first male connector means, and wherein said second slip means is comprised of a first female connector means, said first female connector and said first male connector means attaching said lower cap end to said second slip means, and wherein said upper cap end is comprised of a second male connector means, and wherein said first slip means is comprised of a second female connector means, said second female connector and said second male connector means attaching said upper cap end to said first slip means.
 15. The bridge plug system according to claim 14, wherein said first female connector means and said first male connector means attach in at least one of a group consisting of: slide-fit engagement, groove-fit engagement, friction-fit engagement, and snap-fit engagement, and wherein said second female connector means and said second male connector means attach in at least one of a group consisting of: slide-fit engagement, groove-fit engagement, friction-fit engagement, and snap-fit engagement.
 16. A method of milling a bridge plug system, comprising the steps of: forming a bridge plug, said bridge plug comprising: a mandrel having an upper portion and a lower portion; an upper cap end attached at said upper portion of said mandrel; a sealing member positioned around the mandrel between said upper portion and said lower portion; a plurality of ring members, a first ring member adjacent an upper end of said sealing member and a second ring member adjacent a lower end of said sealing member; a plurality of cone assemblies, a first cone assembly proximate to said first ring member and a second cone assembly proximate to said second ring member, said first ring member being between said first cone assembly and said sealing member, said second ring member being between said second cone assembly and said sealing member; a plurality of slip devices for extending radially outward and engaging an inner surface of a surrounding borehole, a first slip device mounted around said mandrel and engaging said first cone assembly and a second slip device mounted around said mandrel and engaging said second cone assembly; and a lower cap end attached at said lower portion of said mandrel and to said first slip device on a side opposite from said second cone assembly, installing said bridge plug in a wellbore, said wellbore having inner walls surrounding said bridge plug; drilling with a removal assembly through said bridge plug, said upper cap, said upper portion of said mandrel, said sealing member, said ring members, said cone assemblies, and said first slip device, while said second slip device, said lower cap end, and said lower portion of said mandrel are held in position.
 17. The method of milling, according to claim 16, wherein said lower cap end is comprised of a male connector, and wherein said second slip device is comprised of a female connector, said female connector and said male connector means attaching said lower cap end to said second slip device.
 18. The method of milling, according to claim 17, the step of forming said bridge plug comprising: attaching said female connector and said male connector in at least one engagement of a group consisting of: slide-fit engagement, groove-fit engagement, friction-fit engagement, and snap-fit engagement.
 19. The method of milling, according to claim 18, wherein said female connector and said male connector have complementary shapes.
 20. The method of milling, according to claim 17, the step of forming said bridge plug comprising: setting said male connector in said lower cap end by removable engagement of at least one of a group consisting of: slide-fit engagement, groove-fit engagement, friction-fit engagement, and snap-fit engagement. 